Methods of protecting concrete from freeze damage

ABSTRACT

A method of protecting a cementitious mixture from freeze damage is provided. The method consists of incorporating an entrainment air composition into the cementitious mixture to form air voids in the concrete, and further adding an effective agent for nucleating ice, preferably, in the air voids, such that upon the freezing of concrete formed from the cementitious mixture, ice is nucleated in the air voids. In one embodiment, the air entrainment composition includes ceramic shells, which could be impregnated with an agent for nucleating ice such as metaldehyde.

RELATED APPLICATIONS

[0001] This application claims the benefit of provisional applicationU.S. Serial No. 60/131,447 filed Apr. 28, 1999. This application isincorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

[0002] This invention relates to a process whereby the nucleation of icewithin concrete is controlled. It is current practice to protectconcrete against frost damage by introducing air voids, which aregenerated by adding surfactants with the cement paste. Those voidsprotect against one mechanism of damage (hydraulic pressure), but notagainst crystallization pressure. By introducing nucleating agents intothe voids, ice can be forced to occur only in the voids, and this willfurther reduce frost damage.

[0003] Concrete, like all porous media, has the ability to retain andabsorb moisture. Under freezing conditions, ice can grow within theconcrete pores, leading to significant internal cracking of the cementmatrix and/or scaling of the concrete surface. While the precisemechanisms of frost action are not known, concrete deterioration isbelieved to result from three important forces: crystallization,hydraulic and diffusion/osmotic pressures. These mechanisms are thoughtto produce flows of metastable water in the concrete pores that generatesufficiently high stresses to induce fracture of the cement matrix. Toreduce the internal pressures, air-entrained voids are often placedwithin the cement matrix to provide escape boundaries for the flow ofunstable water.

[0004] From experimental evidence, properly air-entrained concretesamples have given consistently good results in terms of the ASTM C 666standard freeze-thaw tests. However, in practice, the technique of airentrainment has several disadvantages such as inconsistencies in spacingfactors (means half-distance between voids) and uncertainties in bubblestability. Both issues have caused frequent discrepancies betweenexpected and actual frost durability

[0005] Numerous references in this area are discussed in the DetailedDescription section of this application.

OBJECTS AND SUMMARY OF THE INVENTION

[0006] It is a primary object of this invention to protect concrete fromfreeze damage.

[0007] It is a further object of this invention to provide a simple,inexpensive, and easy to use method of protecting concrete from freezedamage.

[0008] It is another object of the present invention to add an effectiveamount of nucleating agent to a cementitious mixture to nucleate ice inconcrete.

[0009] It is a further object of the present invention to provide anucleating agent in concrete which can be added during mixing.

[0010] It is even a further object of the invention to provide porousceramic or clay shells for air entrainment in concrete, and to provide amethod of making such shells.

[0011] These objects and others are achieved by the method of protectinga cementitious mixture from freeze damage according to the presentinvention. The method comprises incorporating air into a cementitiousmixture to form air pores in the cementitious mixture, including an airentrainment agent, and adding an effective amount of, preferably,metaldehyde, or an equivalent nucleating compound for nucleating ice inthe air pores upon the freezing of concrete. The nucleating agent isadded to the cementitious mixture during the normal mixing process.Other nucleating agents may be used. Preferably the air entrainmentcomposition contains a surfactant. Because ice nucleating agents arehydrophobic, when mixed with a surfactant, which is normally used forforming air voids, the metaldehyde particles associate themselves withthe surfactant and become incorporated within air voids formed in theconcrete. Optionally, the air entrainment is achieved by using porousceramic shells, which could be used alone or which could be impregnatedwith metaldehyde or another ice nucleating agent. Preferrably, themetadehyde consists of tetrameric units (CH₃CHO)₄, rather thanpolyacetaldehyde chains.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Other important objects and features of the invention will beapparent from the following Detailed Description of the Invention takenin connection with the accompanying drawings in which:

[0013]FIG. 1 is a schematic of approximations of pore geometry in cementpaste in: (a) Straight channels; (b) sloping channels; (c) irregularsized pores with necks that connect to larger pores.

[0014]FIG. 2 is a graph of theoretical minimum pore radius that agrowing ice crystal can penetrate as a function of undercooling.

[0015]FIG. 3 is a graph of tensile stresses induced by crystallizationpressures at increasing undercoolings.

[0016]FIG. 4 is a graph of critical pore length as a function of poreradius and super-undercooling.

[0017]FIG. 5 shows typical air-entrainment compounds used in practicetoday.

[0018]FIG. 6 shows a body-centered tetragonal unit cell of metaldehydeshowing the columnar arrangement of the tetramers.

[0019]FIG. 7 shows an axial view of the packing arrangement of themetaldehydes tetramers revealing the steric effects of the bulky methylgroups.

[0020]FIG. 8 is a graph of DSC comparison of the impregnated andunimpregnated Vycor glass sample.

DETAILED DESCRIPTION OF THE INVENTION

[0021] Structure of Cement and Concrete

[0022] The character of the cementitious porous network is deeplyintegrated with the concrete's freezing properties. Any change in theformer necessarily dictates the behavior of the latter. A large part ofthe reason for uncertainties in frost deterioration stems from thecomplex microstructure of concrete. Due to the nature of the hydrationreaction, voids range from the nanometer (gel pores) to the millimeterscale (capillary pores). Setzer, M. J., “Interaction of Water withHardened Cement Paste” in Ceramic Transactions, Vol. 16: Advances inCementitious Materials, Ed. Sidney Mindness, The American CeramicSociety: Westerville, Ohio, 1990, devised a three-tier classificationscheme that includes the structured surface water, the capillarycondensed water and the macroscopic bulk water. Pore classificationscheme indicating the broad range of pore sizes present in the cementmatrix. Name Upper radius Pore water classification Macro Capillaries 2mm Macroscopic bulk water Meso Capillaries 50 μm Macroscopic bulk waterMicro Capillaries 2 μm Macroscopic bulk water Mesopores 50 nm Condensedwater Micropores 2 nm Structured surface water

[0023] The pore size distribution in concrete is not a fixed parameter.It will vary with chemical composition, aging and the water to cementratio, Mehta, P. K., Concrete: Structure, Properties and Materials,Prentice-Hall, New Jersey, 1986.

[0024] An analysis of the freezing behavior in cement paste will requiresome approximations of the geometry of the porous network. FIG. 1 showssome plausible simplifications of the pore geometry, and shows iceformed therein. As will be discussed in detail, the geometry andconnectivity of the porous network will have a profound influence on thedurability of concrete.

[0025] Freezing within the Pores of Concrete

[0026] It is well known that the freezing properties of a liquid in aporous medium are significantly altered. The phenomenon is due to theinteraction between the liquid (absorbate) and the solid pore surface(absorbent). The nature and intensity of the interactions is a functionof the chemical and geometrical features of the pore wall and of thedistance between the absorbate molecules from the absorbent surface.Collectively, these forces have the effect of depressing the freezingpoint of the pore liquid. Thermodynamic expressions relating thefreezing point depression to the geometry of the pore are wellestablished, Defay, R. and Prigogine, I., Surface Tension andAdsorption, Wiley: New York, 1966; Scherer, G. W., “Freezing Gels,”Journal of Non-Crystalline Solids, V. 155, 1994, pp. 1-25; Marchand. J.,Plea, R. and Gagné, R., “Deterioration of Concrete due to Freezing andThawing,” in Materials Science of Concrete IV, 1991.

[0027] Theoretical considerations imply that the smaller pores will havelower melting temperatures than the larger pores. Thus, the crystal willonly invade the pore when the crystal has undercooled to a lowertemperature, and is able to adopt the required radius of curvature ofthe small pore. Furthermore, theoretical considerations predict thatfreezing will occur in a progressive manner; that is, freezing isinitiated in the larger pores and then penetrates into smaller ones asthe undercooling increases (Scherer, G. W. 1999, “Crystallization inpores”, Cement and Concrete Research, Vol. 29, pp. 1347-1358). This hasbeen confirmed experimentally by various low temperature investigationsof cement paste by either calorimetry or nuclear magnetic resonance(NMR) imaging techniques (Banthia, 1989, Badger, D., “Ice Formation inHardened Cement Paste Part I—Room Temperature Cured Pastes with VariableMoisture Contents,” Cement and Concrete Research, Vol. 16, pp. 706-720;Badger, D., “Ice Formation in Hardened Cement Paste Part II Drying andResaturation on Room Temperature Cure Pastes,” Cement and ConcreteResearch, Vol. 16, pp. 835-844; Badger, D., “Ice Formation in HardenedCement Paste Part III—Slow Resaturation of Room Temperature CuredPastes,” Cement and Concrete Research, Vol. 17, pp. 1-11; Prado, P.,Balcom, B., Beya, S., Bremmer, T., Armstrong, R. and Grattan-Bellew, P.,“Concrete Freeze/Thaw as Studied by Magnetic Resonance Imaging,” Cementand Concrete Research Vol. 28, No. 2, 1998, pp. 261-270). Thisprogressive-freezing phenomenon can be seen graphically in FIG. 2, whichimplies that for an undercooling of ΔT=7° C., ice will not be present inpores smaller than ˜10 nm.

[0028] We can picture a growing ice front initiating at the surface andthen penetrating into the tortuous, interconnected porous network. At agiven undercooling, ice will advance until the ice front is impeded bysmaller pores that require a greater undercooling. The ice front willpercolate through the entire body, only when able to penetrate acritical breakthrough radius, r_(BT). Once entering the breakthroughpore at a characteristic undercooling, ΔT_(BT), the ice front will beable to travel unimpeded in pores of radius≧r_(BT).

[0029] The breakthrough radius, r_(BT) (or equivalently, ΔT_(BT)), isequivalent to the characteristic pore size that controls thepermeability of porous bodies (Katz, A. J., and Thompson A. H., Journalof Geophysical Research, Vol. 92, No. B1, 1987, pp. 599). Thus, highlypermeable materials should have correspondingly larger breakthroughradii than less permeable materials. In many porous materials, ther_(BT) lies near the inflection point in a mercury penetration curve,thus corresponding to the mean pore entry radius. Various r_(BT) withtheir respective ΔT_(BT) (governed by eq. 3.7) are shown in Table 3.2.Breakthrough radius, r_(BT), with respective breakthrough undercooling,ΔT_(BT). Breakthrough Breakthrough radius, r_(BT) [nm] undercooling,ΔT_(BT) [° C.]  5 13.3  10 6.7 15 4.4 20 3.3 25 2.7 30 2.2

[0030] It is difficult to obtain a definitive r_(BT) for cement pastesince the pore structure, and hence the breakthrough conditions, willvary depending on parameters such as the water to cement ratio (w/c),temperature, age and additive concentration. From mercury intrusioncurves generated for cement pastes with a range of w/c ratios, a 0.4 w/cratio paste is expected to have a r_(BT) of ˜15 nm to ˜20 nm.

[0031] The cement paste in concrete will have higher porosities thanplain hydrated paste due to the presence of highly permeable interfacialtransition zones (ITZ) surrounding aggregates (Winslow, D. N., Cohen, M.D., Bentz, D. P, Synder, K. A. and Garboczi, E. J., “Percolation andPore Structure in Mortars and Concrete,” Cement and Concrete Research,Vol. 24, No. 1, 1994, pp. 25-37). The r_(BT) for cement paste inconcrete is therefore higher than the corresponding r_(BT) for plaincement paste. However, common additives for concrete such as silicafume, with particle size ˜3 orders of magnitude smaller than that ofcement particles, will significantly decrease the permeability of aconcrete by reducing r_(BT).

[0032] Crystallization pressure can be defined as the pressure of thegrowing ice crystal on the pore wall. Theoretical calculations andconsiderations, as shown in FIG. 3, indicate that crystallizationpressures will potentially exceed the concrete tensile strength (about 3MPa) at undercoolings greater than or equal to 5° C. and pore radii lessthan or equal to 13.3 nm.

[0033] To cause fracture, the generated tensile stresses must act on theflaws in the pore wall. At the breakthrough temperature, T_(BT), most ofthe pore volume has frozen, implying that all of the flaws (includingthe most damaging large flaws) in the body should feel the stress atthat point. Hence, crack propagation is expected to strongly correlatewith the propagation of the ice front at T_(BT).

[0034] After ice has propagated and cracking has initiated in the body,the effect of crystallization pressures at lower temperatures important.As long as there is water in contact with the ice, crystals will form atsufficiently low temperatures, and crystallization pressures will bepresent. There are generally small isolated pockets of unfrozen watereven after percolation. Thus, when the temperature drops below T_(BT)there is crystallization pressure as the ice front is penetrating intothe smaller pores of the unfrozen pockets. The generated stresses couldbe quite high (>10 Mpa) but likelihood of failure is dependent on thewhether stresses are brought to bear on flaws in the small pores.

[0035] It is highly probable that the percolation event at T_(BT) willaccelerate crystallization pressure damage by amplifying stresses on thelargest flaws in the body. It is desirable that nucleation in the airvoids occur at a temperature above the T_(BT); in that way, thefreezeable water is removed from the pores before stresses are appliedto the largest flaws in the body. Moreover, it is preferred thatnucleation occur above about −5° C., thereby confining the ice growth tothe air voids before stresses can theoretically exceed ˜3 MPa.

[0036] Water has the unusual property that the liquid phase (ρ=1.00g/cm³) is more dense that the solid phase (ρ=0.92 g/cm³). This propertyhas very important repercussions in the freezing of porous media sinceice necessarily undergoes a 9% expansion. The volume change forces waterahead of the growing crystal thus creating a pressure gradient in thepore.

[0037] Based on theoretical consideration, FIG. 4 shows the maximumlength the displaced water can travel before generating tensile stressesexceeding that of concrete. Thus, if an ice crystal is growing in a 10nm pore with a ΔT* of 1° C., cracking of the pore wall will potentiallyoccur if the displaced water does not reach an escape boundary by thetime it travels ˜470 μm. FIG. 4 also suggests that ice growth in smallpores with large super-undercoolings will be the most damaging (i.e.,having the lowest critical pore lengths). Furthermore, with increasingΔT*, the slope of the critical pore length curve will necessarilydecrease, causing larger pores to enter into the probable vulnerablezone. As the percentage of the “nondurable pores” increases, theresistance to hydraulic pressure damage necessarily decreases.

[0038] A crystal front must be growing (possessing a finite velocity)for hydraulic pressures to generate. However, if the temperature has notreached the breakthrough conditions, the ice front will most likely bein a stationary state, pressing against the pore walls, and hence,creating negligible hydraulic pressures. Only when the breakthroughconditions have been met (namely ΔT_(BT) and r_(BT)) will an ice frontbe able to grow for extended lengths, and thereby create significanthydraulic pressures. Now, we can picture a crystal just penetrating apore of size r_(BT) at an undercooling of ΔT_(BT). As the crystal frontpercolates through a network of pores with radii greater than or equalto r_(BT) at a temperature of T_(BT), the super-undercooling,ΔT*=T_(BT)−T, will change depending on the size of the pore. Hence, theice front will pulse along the percolation path with the greatestvelocities occurring in the larger pores. Moreover, since the tensilehoop stress in the pore wall is a function of ΔT* (power dependence of1.7) and r_(p) (inverse square dependence), the stresses generated willalso vary with changes in pore sizes.

[0039] The freezing rate in nature is low with a maximum rate of 6°C./hr. Thus, one can reasonably assume that the temperature remainsconstant at the breakthrough temperature, T_(BT), throughout the entirepercolation event. For a modest super-undercooling of ΔT*=1.0° C. atbreakthrough conditions (thus the front is free to travel longdistances) an ice front can move ˜6 m in one hour. With this speed,concrete slabs will fully crystallize before the temperature drops farbelow the breakthrough temperature, T_(BT).

[0040] Theoretical considerations predict the damaging effect ofinducing hydraulic pressures in a very fine porous network (possessing asmall characteristic r_(BT)). For a r_(BT) of 10 nm, stresses canapproach devastating stresses of ˜30 MPa when invading larger pores.Furthermore, such theoretical considerations contradict the idea thatconcrete will necessarily be less prone to frost damage if it is lesspermeable. It is certainly true that concrete will be completelyprotected if there is absolutely no freezeable water present in thepores. However, this level of impermeability is very difficult toachieve even with very fine porous networks. Moreover, theoreticalconsiderations confirm the generally accepted notion that high strength(compressive strengths>˜100 MPa) and silica fume concretes are verysusceptible to damage in winter climates (Mehta, P. K.,“Durability—Critical Issues for the Future,” Concrete International,July, 1997, pp. 27-33). Most likely this susceptibility is due to thelow characteristic breakthrough radii for these very fine concretes.Maximum tensile stresses generated from hydraulic pressures for aspecific r_(BT) with the assumption that the maximum pore size is ˜50nm. Breakthrough radius, r_(BT) [nm] Maximum tensile stress [MPa]  5433.6  10 27.3  15 4.9 20 1.3 30 0.1

[0041] As mentioned earlier, the r_(BT) for cement paste in concreteshould vary from sample to sample depending on the curing environment.Assuming a maximum pore size of 50 nm as before, it can be calculatedthat stresses are expected to be greater than 3 MPa only when r_(BT) isless than about 16.7 nm. The r_(BT) was estimated for plain cementpastes to be ˜15 nm to ˜20 nm, so it is not unreasonable to assume thatconcrete can possess a r_(BT) of 16.7 nm, which would imply potentiallydamaging tensile stresses.

[0042] Up to this point, it has been assumed that ice is present in thecement pores prior to reaching the breakthrough temperature.Equivalently, this assumption implies that heterogeneous nucleation hasoccurred near 0° C., presumably at the surface where foreign catalystsare most probable. While there have not been any extensive studies onsurface nucleation for concrete in the environment, there have been somestudies on laboratory quality samples. Calorimetry experiments by Badgerand Banthia et al. revealed an initial freezing peak near −10° C. andeither one or two peaks between −20° C. to −40° C. Both papers agreethat the initial peak corresponded to the nucleation of the ice at thesurface of the sample. Badger further cited that the initial peak couldbe shifted towards higher temperatures by adding AgI (effective icenucleant at T=˜−4° C.) on the surface of the sample. Of course, thelaboratory samples studied in these calorimetry experiments willprobably not contain potential natural ice nuclei such as some activebacteria which are known to induce crystallization on non-coniferousplants as high as −2° C. (Vali, 1971). However, it is also doubtful thatthere will always be a high enough concentration of these effectivenucleating agents on exposed concrete surfaces.

[0043] Thus, if a concrete surface is “clean,” nucleation on the surfacecould very well be delayed to −10° C. as in the laboratory samples. Themost important implication of a delayed surface nucleation is thatpercolation can now occur at temperatures lower than T_(BT). Ifnucleation occurs at a temperature, T_(N), which is lower than T_(BT) ,the percolation event can occur at the lower temperature, leading tohigher ΔT*, and hence, higher tensile stresses.

[0044] Besides the initial expansion of concrete at the onset offreezing, concrete undergoes considerable shrinkage during freezing ifheld at a constant sub-zero temperature. It has been hypothesized thatthe ice crystals that were initially formed in the larger pores couldfeed off the unfrozen water in the neighboring nanosized gel pores(Powers, T. C. and Helmuthm, R. A. 1953, “Theory of volume changes inhardened portland-cement paste during freezing”, Proc. Highway Res.Board, Vol. 32, pp. 285-297). This ice accretion mechanism is thought tobe a result of the free energy gradient between the crystal and theunfrozen gel water. At the onset of crystallization, the ice and gelwater are in equilibrium. As the undercooling increases, the gel water(having greater entropy) should gain free energy at a faster rate thanthe crystal. Thus, to regain equilibrium, the gel water migrates to thegrowing crystal and is allowed to shrink.

[0045] Osmotic pressure theories were later added to account for theshrinkage of concrete during prolonged freezing periods. The origin ofosmotic pressures is that salt is highly insoluble in ice. Consequently,very steep salt gradients accumulate at the ice/water interface.Moreover, since ice will tend to initiate near the surface of concretestructures (as a result of minimum temperatures), the highest saltgradients should occur near the surface. Amplifying the effect is theuse of surface deicer salts on concrete roads. The net result is amigration of the dilute gel water to the high salt concentration at thesurface and shrinkage of the interior concrete layers. The combinationof the shrinkage of the interior gel layers and the expansion fromfreezing in the saturated surface layer produces potentially destructivestresses.

[0046] Air Entrainment Agents

[0047] Introducing a nucleating agent directly in the air voidsinitiates ice growth in the large air voids and minimizes the internalpressures created by the metastable water (whether from hydraulic ordiffusion/osmotic mechanisms).

[0048] The purpose of an air-entrainment agent is not to entrain airbubbles, which is done mechanically in the mixer, but to stabilize thebubbles in the cement matrix. The role of the air-entrainment moleculesis to stabilize the air-water interface, reduce the surface tension ofwater (by as much as ˜20%), and to bind the air bubbles to the cementparticles. Most air-entrainment compounds are aqueous solutions of ionicor nonionic surfactants, implying the presence of hydrophilic heads andhydrophobic tails. Air-entrainment molecules stabilize air bubbles byadsorbing at the air/water interface with their hydrophobic endsprotruding into the air-void itself and their hydrophilic ends remainingin the aqueous phase.

[0049] Commercial air-entrainment products are typically dilute aqueoussolutions (5% to 20% by weight) of surfactants (Rixom, M. R. andMailvaganam. N. P., Chemical Admixtures for Concrete, E.&F.N. Spon.:London, 1986). In practice, there are five basic groups of surfactantssuitable for concrete use (shown in order of probably decreasing use):

[0050] (a) Abietic and pimeric acids salts (neutralized wood resins)

[0051] (b) Fatty acid salts

[0052] (c) Alkyl-aryl sulphonates

[0053] (d) Alkyl sulphates

[0054] (e) Phenol ethoxylates.

[0055] The chemical structure of a representative of each group can beseen in FIG. 5.

[0056] What is interesting about the different air-entrainment compoundsseen in FIG. 5 is that they possess varying degrees of freeze-thawresistance at a given total air-content. This implies that there is achemical interaction taking place in the air-voids between theair-entrainment and water molecules. Kreijer. C. I., “Effect ofAdmixtures on the Frost Resistance if Early-Age Concrete,” in RILEM-ABEMInternational Symposium on Admixtures for Mortar and Concrete, Brussels,pp. 235-244, 1967, showed that for an air content of ˜5%, sodium oleateproduced the best freeze-thaw resistance while phenol ethoxylate showedvery little improvement over the non-air-entrained control sample. Thereasoning for this discrepancy is that an air void can obviously notserve as a sink for displaced water if it is already full of water. Thewater-free void will be ensured if the tails of the air-entrainmentmolecules are highly hydrophobic. This explains partly whynon-hydrophobic molecules such as phenol ethoxylate (capable ofhydrogen-bonding on oxygens) yield poor freeze-thaw results while thehydrophobic oleaetes, sulphates and resins perform much better. Varyingfreeze-thaw resistance of several air-entrainment admixturers. DosageAir Relative (m./50 kg content freeze-thaw Air-entrainment admixturecement) (%) Resistance¹ None (control)  0 2.0  5 Sodium oleate (10%sol.) 353  5.6 86 Sodium lauryl sulphate 18 5.8 46 Pine resin 15 5.2 57Phenol ethoxylate 75 5.2  7

[0057] A discussion of the chemistry of air-entrainment molecules waspresented since it is desired to introduce ice nucleating agents intothe air voids. When selecting air-entrainment compounds, the chemicalinteraction between these molecules and the ice nuclei should beunderstood. A nucleating particle is thought to contain active sites andplanes where nucleation is favored. If these active sites stronglyinteract with the air-entrainment molecules and “poison” the nucleatingsurface, the activity of the nucleating particle will correspondinglydecrease. However, since it is desired to concentrate the nuclei in thevoids and not in the cement pores, there needs to be some attractiveforces present between the nuclei and air-entrainment molecule to ensurethat the two settle in the voids after hydration. This attractive forcemust be sufficient to maintain the bond between the two compounds evenafter mixing of the concrete. But again, it must be remembered that theattractive force should not annul the nucleating properties, implyingthat a compromise must be established.

[0058] Nucleation of Ice

[0059] Although ice melts consistently at 0° C., pure liquid water cansupercool to as low as −40° C. (Weissbuch, et al.; and Hobbs). Theinduction, or catalysis, of the freezing point to higher temperatureshas many important consequences in nature. Vonnegut, B., Journal ofApplied Physics, Vol. 18, 1947, pp. 593. was the first to identify thatsilver iodide could induce ice nucleation in atmospheric clouds at ˜−4°C. His finding has spurred much research on other inorganic, organic(Fukata N., “Experimental Studies of Organic Ice Nuclei,” Journal of theAtmospheric Sciences, Vol. 23, 1966, pp. 191-196; Garten, 1965) andbacterial nuclei. (Maki. L. R., Gaylam, E., Chang Chien, M. andCaldwell, D. R., Applied Microbiology, Vol. 28, pp. 456-459;Gurian-Sherman, D. and Lindlow, S. E., “Bacterial Ice Nucleation:Significance and Molecular Basis,” The FASEB Journal, Vol. 7, November1993, pp. 1338-1343; Wolber, P. K., “Bacterial Ice Nucleation,” Advancesin Microbial Physiology, Vol. 34, 1993, pp. 203-237; Pattnaik, P.,Batish, V. K., Grover, S. and Ahmed, N., “Bacterial Ice Nucleation:Prospects and Perspectives,” Current Science, Vol. 72, No. 5, Mar. 10,1997, pp. 316-320. Fukuta, N., “Ice Nucleation by Metaldehyde,” Nature,Vol. 199, 1963, pp. 475-476; Fukuta, N., “Some Remarks on Ice Nucleationby Metaldehyde,” in Proceedings of the International Conference on CloudPhysics, Aug. 26-30, 1968, Toronto, pp. 194-198, found that metaldehydenucleated ice as high as −0.4° C. from the vapor phase. Frost inducingbacteria (Vali, 1971) was discovered to be the cause for much of thewide spread damage to nonconiferous plants due to nucleation of ice ashigh as −2° C. Recently, much research has been devoted to thenucleating properties of monolayers of amphiphillic alcohols(C_(n)H_(2n+1)OH). It has been found that C₃₁H₆₃OH (n=31) could nucleateice as high as −1° C. (Gavish, M., Popovitz-Biro, R., Lahav, M. andLeiserowitz, L., “Ice Nucleation by Alcohols Arranged in Monolayers atthe Surface of Water Drops,” Science, Vol. 250, 1990, pp. 973-975;Popovitz-Biro, R., Wang, J. L., Majewski, J., Shavit, E. Leiserowitz, L.and Lahav, M., “Induced Freezing of Supercooled Water into Ice bySelf-Assembled Crystalline Monolayers of Amphiphillic Alcohols at theAir-Water Interface,” Journal of the American Chemical Society, Vol.116, 1994, pp. 1179-1191).

[0060] The basis of the theory of nucleation of new phases wasestablished long ago by Volmer M. and Weber, A., Zeitschrift fuerPhysikalische Chemie, Vol. 119, 1325, pp. 277 and Becker, R., andDoring, W., Annalen de Physik (Leipzig), Vol. 5, No. 24, 1935, pp. 719,and remains virtually unchanged today. Within a supercooled liquid or asupersaturated vapor, there are transient groupings of the parentmolecules with the structure of the stable phase (ice, in this case).These fortuitous embryos are unstable and are continuously being createdand destroyed by thermal fluctuations in such a fashion that a Boltzmandistribution in energy is maintained. The free energy barrier (ΔG*)associated with the formation of a stable embryo has a maximum value ata certain critical embryo size. Once the embryo reaches this size (orequivalently, when the embryo contains a critical number of watermolecules) crystallization occurs spontaneously.

[0061] There are two mechanisms of ice nucleation commonly recognized(Fletcher, N. H., “Chemical Physics of Ice,” Cambridge University Press,Cambridge, 1970, pp. 73-103). Homogenous nucleation refers to thespontaneous nucleation of ice crystallization in pure supercooled water.Nucleation by this mechanism requires overcoming a high free energybarrier due to the large surface free energy requirements. Heterogeneousnucleation involves the binding of supercooled water molecules toforeign particles to initiate nucleation. The presence of the particlespromotes nucleation since it reduces the surface energy investment, andhence, the free energy barrier to nucleation.

[0062] Since homogeneous ice nucleation can only take place attemperature below −35° C. (Franks, 1985), most ice transformations innature must occur by heterogeneous nucleation. This phenomenon can beexplained by the high probability for foreign particles in naturallyoccurring liquid or vapor phases. An efficient nucleating agent is onethat has a good lattice match, or structural fit, with the ice crystal(Fletcher, N. H., “Nucleation and Growth of Ice Crystals UponCrystalline Substrates,” Australian, Journal of Physics, Vol. 13, 1960,pp. 108-419; Fletcher, N. H., “Chemical Physics of Ice,” CambridgeUniversity Press, Cambridge, 1970, pp. 73-103). Fletcher, N. H. hasdiscussed several other factors affecting nucleation, including thecontact angle (between the ice crystal and substrate), nucleant size(Fletcher, N. H., “Size Effect in Heterogeneous Nucleation,” The Journalof Chemical Physics, Vol. 29, No. 3, 1958, pp. 572-576), effects oftopographical imperfections (Fletcher, N. H., “Active Sites and IceCrystal Nucleation,” Journal of the Atmospheric Sciences, Vol. 26, 1969,pp. 1266-1271) and an entropic consideration of the induced dipoles inthe water molecules (Fletcher. N. H., “Entropy Effect in Ice CrystalNucleation,” The Journal of Chemical Physics, Vol. 30, No. 6, 1959, pp.1476-1482).

[0063] The two most well-known nucleating substrates for ice are AgI andPbI₂ (identified by Vonnegut).

[0064] Experimental results show a wide range in the onset temperaturesfor various nuclei. The activity spectrum can be attributed to thedistribution of “active sites” upon the surfaces of the nucleatingparticles (Fletcher, 1969). Active sites refer to the particular siteson the nucleating particle that have the highest probability of forminga stable embryo. For clarification, when saying that AgI has an onsetnucleation temperature of −4° C., we are actually quantifying thenucleating ability of the active sites. Not all particles are active at−4° C.; indeed, to achieve 100% activity for AgI particles≧100 Å inradius, one must lower the temperature to −22° C. (Mussop S. C. andJayaweera, K. O. L. F., “AgI-NaI aerosols as ice nuclei,” Journal ofApplied Meteorology, Vol. 8, pp. 241-248).

[0065] There are several general requirements for an efficient activesite. First, the contact area between the embryo and the nucleus must becomparable with the total surface area of the embryo if the nucleus isto be effective. If the low energy site is too small, only a few watermolecules will be captured and the resulting embryo will not be stable.Fletcher (1969) estimates that freezing nuclei must be on the order of200/ΔT [Å] if nucleating at −ΔT [° C.]. Also important is that theinterfacial free energy of the particle-ice interface must be as low aspossible. Thus, the chemical nature is clearly important since itdictates the bonding between the substrate and overgrowing ice crystal.The crystallographic nature of the substrate has an equally importantrole in the energy of the interface due to the specific alignment of thesurrounding water molecules.

[0066] The residual entropy of the ice crystal structure influences thenucleation process. Fletcher (1959) was the first to explore theconsequences of the randomly oriented water dipoles and claimed thatthere is an entropic penalty if a heterogeneous catalyst orients thedipoles parallel to the catalyst surface. The reasoning is that if thedipoles are ordered (as would be the case on a surface of uniformcharge) the entropy of the ice structure would be reduced and theresulting free energy barrier would increase. Consequently, theuniformly charged surface would be a poor nucleating agent and require alarger undercooling for inducing crystallization.

[0067] From these theories, Fletcher predicts that the uniformly charged(either +1 or −1) basal surfaces {0001} of AgI and PbI₂ should be poornucleating planes as a result of the entropic penalty. However, theprism faces of these crystals, having an equal distribution of positiveand negative charges, will not orient the dipoles parallel to thesurface (probably in the plane of the surface) and hence, be betternucleating planes. Another implication of Fletcher's theory is that thesteps on basal and prism planes are not necessarily equivalent in termsof nucleating activity. Steps on basal planes, exposing prism faces, aregood nucleating sites while steps on prism faces expose basal planes,and hence, are not expected to be good nucleating sites.

[0068] Fletcher's predictions were first confirmed experimentally byEdwards, L. F. and Evans, G. R., “Effect of Surface Charge on IceNucleation by Silver Iodide,” Trans. Faraday Soc., Vol. 58, pp.1649-1655, who found that AgI was most active at its isoelectric point.Isono, K. and Ishzaka, Y., Journal de Recherches Atmospheriques, Vol. 6,1972, pp. 283, showed that the (111) face of pure γ-AgI and the (1010)face of pure β-AgI (where both Ag⁺ and I⁻ are present) were more activethan the (0001) face of β-AgI (where either Ag⁺ or I⁻ are present).Pruppacher, et al., (1975) etched a ferroelectric substrate creatingadjacent positively and negatively charged domains and founds that icepreferentially nucleated on the boundaries rather than within auniformly charged domain. It was concluded that by nucleating on theboundaries between the domains, the water molecules could randomlyorient in the plane of the substrate, thereby eluding Fletcher'sentropic penalty.

[0069] From overwhelming experimental evidence, it is seen that the mostefficient ice nuclei are insoluble in water. Fukata (1958) showed thatwater soluble salts with ice-like lattice parameters (such as CdI, NH₄F,CaI) could not exceed onset temperatures higher than ˜−11° C. Thedifficulty in nucleation is thought to be a result of the instability ofthe ice embryo caused by the diffusion of water molecules and substratemolecules across the embryo surface. For this reason, it was thoughtthat an efficient nucleus should be hydrophobic in nature. Thehydrophobic surface can be thought of as forcing the surrounding watermolecules into an “uncomfortable” state, thereby making crystallizationenergetically more favorable than the supercooled phase. Furthermore,the increased energy of the substrate/liquid interface (γ_(SL)) willreduce the contact angle with the ice crystal and thereby favornucleation.

[0070] While most of the classical theories of heterogeneous icenucleation were developed with inorganic compounds such as AgI, there isresearch on organic crystal nuclei from the 1960's. A major incentivefor using organic nuclei is the possible lower costs when compared tothose of inorganic nuclei. Fukata (1963,1966) tested 329 organiccompounds and found that metaldehyde could nucleate ice as high as −0.4°C. when exposing particles (less than 13μ in diameter) to water vapor.Six other compounds, acetoacetanilide, p-bromoacetphenone, coumarin,m-nitroaniline, phtalic anhydride, 2,4,6-trichloroaniline showed icenucleation thresholds almost as high as metaldehyde (−1.5° C. to −1° C.)when exposing the freshly ground samples to water vapor.

[0071] One of the main differences between organic and inorganic nucleiis the fact that the former can participate in hydrogen bonding with thewater molecules. Head, R. B., Journal of Physical Chemistry—Solids, Vol.23, 1962, pp. 1371, was the first to show that hydrogen-bonding isessential for organic ice nucleation. Garten, et al., (1965) expressesthe idea of hydrogen bonding group (HBG) density by implying that themost efficient organic nuclei have HBG densities (3-4 per 100 Å²) whichare lower than those of ice (5-7 per 100 Å²). When the density on anyplane exceeds the latter figure, the substrate becomes hydrophilic, andfor reasons expressed above, the nuclei become ineffective. Garten lateradds that the effect of excess HBG's is to stabilize the denserstructure of liquid water rather than that of ice to higherundercoolings. Molecular symmetry, as Fukata (1966) points out, is alsoimportant. It was claimed that organic molecules with rotationalsymmetry are better nuclei than non-symmetrical molecules since theformer cannot avoid exposing their active HBG's at the surface.Non-symmetrical molecules, on the other hand, will tend to point theirHBG's inward since the hydrogen bond is energetically costly and aminimum surface free energy is desirable. Consequently, the contactangle between the ice embryo and the HBG's on the non-symmetricalmolecules will be high, inhibiting nucleation.

[0072] A preferred nucleating agent of this invention is metaldehyde.Metaldehyde (CH₃CHO)₄, the cyclic tetramer of acetaldehyde, stillpossesses the highest nucleation temperature for crystalline substancesat −0.4° C. (Fukata 1963). The structure, as deduced by Pauline, L.,Journal of the American Chemical Society, Vol. 57, pp. 2680, producessome interesting crystal properties. First, the tetragonal latticeparameters of a₀=10.40 Å and c₀=4.11 Å of the unit cell are close tothat of ice, hence making it a potential ice nucleant. The puckered8-member ring (C-O distance=1.43±0.03 Å, C-C distance=1.54±0.03 Å)creates a negatively charged plane of oxygens (facing down) and apositively charged plane of hydrogens (facing up). Consequently, whenpacking these tetramers, the molecules will stack in weakly-interactingcolumns oriented in the c-directions. This can be understood by the factthat the oppositely charged faces cause strong attractive forces in thec-direction while the inactive, bulky methyl groups shield the columnsfrom each other. The methyl groups are approximately equidistant fromeach other, having two groups a distance of 3.90 Å away, four at 4.03 Åand two at 4.11 Å (directly above and below). This arrangement yields apacking radius for the methyl groups of 2.01±0.06 Å. As a result of thesteric effects of the methyl groups, metaldehyde will form long bundlesof easily cleaved fibers if allowed to recrystallize slowly in anysuitable solvent. (THF or chloroform). A body centered tetragonal unitof metaldehyde is shown in FIG. 6, and an axial view of the packingarrangement of metaldehyde tetramers is shown in FIG. 7.

[0073] Another interesting phenomenon is the fact that the c_(o)/2translation in each column relative to its four nearest neighbors bringsthe molecular dipole into an electrostatically stable configuration inwhich the oppositely charged poles are arranged as nearest neighbors.

[0074] Fukata (1968) refers to this dipole stabilization as apyroelectric effect. It was further cited that this effect is beneficialin cloud seeding since it allowed the metaldehyde smoke particles toinduce polarization and attraction of the water droplets.

[0075] Metaldehyde, like other organic and inorganic (including AgI)nuclei, is known to be photosensitive. Fukata (1963) showed that afterexposing metaldehyde to sunlight for more than one hour at temperatureshigher than 55° C., the nucleating property completely disappears. Theexact cause of this phenomenon has not been confirmed. It may be due tosome free-radical, photo-oxidative process as exhibited by phologlucinoland α-phenazine (Garten 1965). The vanishing nucleating properties couldalso be due to the decomposition of metaldehyde to paraldehyde (cyclictrimer of acetaldehyde) at 80° C. Regardless of the cause thetemperature range of the specific nucleation application for metaldehydeneeds to be taken into account.

[0076] Metaldehyde (and organic crystals in general) have lost theirresearch appeal as heterogeneous nuclei since the 1960's and have givenway to inorganic AgI-based compounds. One explanation is thatmetaldehyde, while much cheaper than AgI and better in terms of onsetnucleation temperature, is not as active as AgI at higher undercoolings;specifically, the number of active nuclei per gram of substrate isalmost four orders of magnitude lower for metaldehyde than AgI (Garten1965). This relative specific inactivity in metaldehyde is probably morea concern in cloud seeding (which was the application in mind) sinceindividual smoke particles have to nucleate independently. Anotherpossible reason for the decline in interest in metaldehyde (and otherorganics) is its toxicity problem. Metaldehyde is widely used as snailpoison and may pose a risk to humans if consumed (Morgan D. P.,“Miscellaneous Pesticides, Solvents, and Adjuvents,” in Recognition andManagement of Pesticide Poisonings, 4^(th) ed., Chapter 15,Environmental Protection Agency, March 1989). This toxicity issue wouldbe important if metaldehyde were used as a cloud seeder (as it wasintended to be); however, if immobilized in a cementitious matrix, thetoxicity issue becomes less important. If metaldehyde did spread tonatural environments, contamination would be minimal since metaldehydewould depolymerize to acetaldehyde and oxidize eventually to harmlesslevels of acetic acid.

[0077] In order to place the metaldehyde in the air voids, thecommercial grade metaldehyde particle size has to be reduced, becausethe average air void is roughly 100 microns in diameter. In addition, amechanism for transporting the metaldehyde into air voids had to beestablished. To reduce the particle size we initially tried ball millingthe metaldehyde. However, the ball-milled metaldehyde showed noeffective nucleating abilities. Next we tried grinding the metaldehyde,and found that the ground metaldehyde effectively nucleated freezingaround −1° C.

[0078] To account for the difference between the ball-milled and theground metaldehyde's nucleating ability, we performed x-ray diffractionon each of the samples. The results illustrate a predominance of themost effective nucleating plane (viz., the (110) crystallographic plane)exposed in the ground sample, while the milled sample showed a moreuniform distribution of exposed planes. This result leads to theconclusion that vigorous ball-milling randomized the exposed planes,while the subtle grinding catalyzed cleavage predominately along thenucleating plane. In addition, this result confirms Fukata's suggestionthat the (110) plane is the effective plane of nucleation.

[0079] Two types of metaldehyde-containing cement paste samples wereproduced: metaldehyde with and without air entrainment. Incorporation ofmetaldehyde into the air voids could only be accomplished in the samplewith air entrainment. To accomplish this deposition, the air entrainmentagent is thoroughly mixed with the ground metaldehyde before adding thesolution to the cement paste.

[0080] The influence of metaldehyde on the freezing behavior of thecement paste was evaluated using the DMA (Dynamic Mechanical Analyzer).The DMA measures dilation in a sample as a result of freezing. Afteranalyzing both samples it appeared that the metaldehyde workedeffectively: samples with and without air showed a higher freezing pointand gradual dilation. For reference a plain paste sample was also run,and showed an abrupt dilation at roughly −9° C. The elevated freezingpoint shows that metaldehyde is effectively nucleating freezing around−3° C.

[0081] Effectiveness of Air Entrainment

[0082] The effectiveness of entrained air voids in protecting concretefrom freeze/thaw damage is well known. According to most microscopictheories of frost action, air voids act as escape boundaries for therejected water, whether originating from hydraulic, diffusion or osmoticmechanisms. Properly entrained concrete has shown good resistance tointernal cracking and scaling in the laboratory under standard ASTM C666 freeze-thaw tests which subject samples to 300 freezing and thawingcycles typically at a rate of 6° C./h to 8° C./h.

[0083] Experiments have also shown that the most reliable measure forfrost protection is the air void spacing factor, {overscore (L)};although recognized as a useful parameter, it is not easy to measure.The main cause for this difficulty arises from the fact that air-voidsare randomly distributed in the cement paste. Microscopical evaluationof polished concrete samples (as outlined in “Standard Practice formicroscopical determination of air-void content and parameters of theair-void system in hardened concrete,” Annual Book of ASTM Standards,ASTM, Philadelphia, Pa., 1990) is the only direct measurement of thespacing factors; however, this procedure is time consuming and certainlynot applicable for on-site assessments. It would be ideal if there werea way to predict and consistently control the air-void network. CurrentASTM C 457 procedure provides simple equations as guidelines but theytend to grossly oversimplify the random air-void network.

[0084] Numerous studies have been conducted to better approximate therandom distribution of air voids. Attiogbe, E. K., “Mean Spacing ofAir-Voids in Hardened Concrete,” ACI Materials Journal, Vol. 90, No. 2,March.-April. 1993, pp. 174-181; Attiogbe, E. K., “PredictingFreeze-Thaw Durability of Concrete—A new Approach,” ACI MaterialsJournal, V. 93, No. 5, 1996, pp. 457-464, for example, accounts for therandomness of the voids by coupling the idea of a mean factor,{overscore (s)}, and the parameter F, which represents the fraction ofthe total paste volume within the radial distance {overscore (s)} fromthe edges of the air-voids. However, it is questionable whether anymathematical model can accurately account for the instabilities that areinherent with entraining air in concrete.

[0085] Despite fairly good predictability of freeze-thaw behavior in thelaboratory, concrete structures still suffer from frost damage inpractice. The problem of air-entrainment can essentially be reduced totwo questions: (i) What are the required air void characteristics (e.g.,spacing factors) for the specific concrete system in use? (ii) How canthe air-void system be preserved in a reliable and consistent mannerduring the setting of cement?

[0086] In terms of the first question, the chemistry of the cementbinder has a direct influence of the pore structure, and hence, thefreezing properties of the concrete. As previously discussed, thepermeability (quantified by r_(BT)) will significantly affect theresistance to induced internal pressures. The lower the permeability,the more resistance the displaced water will experience, and thus,higher stresses are generated. To compensate for low-permeabilityconcretes made using admixtures such as water-reducing agents and silicafume, a greater air content must be entrained in the concrete to ensurea shorter spacing length. This practice is disadvantageous in two ways.First, increasing the entrained air necessarily decreases the strengthof the concrete, thus, possibly negating the benefits of the admixturesaltogether. Second, and probably more important, is that the spacingfactor will vary depending on the characteristics of the cement paste inthe concrete. This makes standardization of building practices verydifficult, if not impossible.

[0087] Taking all types of cements into consideration, it is generallythought that a spacing factor of 200-250 μm represents an adequatelyfrost resistant concrete (Marchand). As mentioned earlier, the realproblem is the reproducibility of generating an air-void network with adesired spacing factor. Since there is no direct technique to measurespacing factors onsite, our national standards only require themeasurement of total air content (a readily measurable parameter) ratherthan spacing factors. It is generally believed that air contents in therange of 5% to 8% by volume correlate with frost protected spacingfactors (on the order of 200 μm). However, experience has shown thatthis assumption is not a valid one. In fact, it was seen that spacingfactor can vary considerably with a given air content. Specifically, a6% air content can yield a spacing factor of 100 μm to 400 μm (Saucier,F., Pigeon, M. and Cameron, G., “Air Void Stability—Part V: Temperature,General Analysis and Performance Index,” ACI Materials Journal, Vol. 88,1991, pp. 25-36). This discrepancy could easily make the differencebetween a durable, frost resistant concrete and a frost prone concrete.

[0088] The reasons for the poor correlation between air content andspacing factor are believed to result from three sources of bubbleinstability: buoyancy, coalescence and dissolution effects. The neteffect of these instabilities result in an increase in the spacingfactors, and hence, a decrease in frost resistance. Buoyancy effectsrefer to the phenomenon of the tendency for larger bubbles to rise tothe surface and to be expelled from the paste. This phenomenon resultsfrom the fact that the buoyancy force (proportional to volume, πd³/6)for larger bubbles can overcome the shearing frictional force(proportional to πd). The coalescence of air bubbles results from thedrive to reduce the free energy of the system by decreasing theinterfacial surface area of the bubble. For two identical volume airbubbles with the same surface tension, the coalescence of the twobubbles result in 21% reduction in energy of the two bubble system(Marchand). Dissolution of air is the third source of instability and itcomes from the fact that the solubility of air increases with pressure,and the pressure inside an air bubble is inversely proportional to itsdiameter (Kelvin's law). Thus, small air bubbles have a tendency tocollapse due to the solubility effect.

[0089] These bubble instabilities are even more pronounced during thetransportation and pumping of concrete where significant air losses canoccur. It would be ideal if a void system could be produced in concretethat has a controllable spacing factor. Moreover, it would beadvantageous to minimize the spacing factor to ˜100 μm or less. This isnot a feasible solution for the air entrainment technique since theinclusion of more air voids will necessarily reduce the strength ofconcrete and allow for further cracking of the matrix.

[0090] To avoid the problems of instability and unreliability of airvoids produced using air-entrainment agents, we prefer to use porousceramic shells for air entrainment. The preparation and use of suchshells is described in the following section.

[0091] Hollow Shells as Alternative to Air Entrainment

[0092] Hollow shell technology has attracted attention in applicationssuch as low dielectric constant materials, fiber-optic microsensors,light weight composites and impact resistant materials (Wilcox, D. andBerg, M., “Microsphere Fabrication and Applications: An Overview,” inMaterials Research Society Symposium Proceedings, Vol. 392, Hollow andSolid Spheres and Microspheres: Science and Technology Associated withTheir Fabrication and Application, 1994, pp. 3-13). One method ofpreparing well-shaped hollow and spherical particles is by spraypyrolysis (SP). The technique differs from the well-establishedtechnique of spray drying in the use of solutions (rather thanslurries), the process of precipitation or condensation within thedroplet, and the use of significantly higher temperatures (˜>300° C.).During SP, the solution is continuously atomized in a series of reactorswhere aerosol droplets experience solvent evaporation and solutecondensation within the droplet, drying, thermolysis of the precipitateto form a microporous particle, and finally sintering to achieve fulldensity.

[0093] Either solid or hollow spheres can result from SP depending ondroplet size, solute concentration, precursor supersaturation andevaporation rate (Charlesworth, D. and Marshall, W., Journal of theAmerican Institute of Chemical Engineering, Vol. 6, No. 9, 1960; Leong,K., Journal of Aerosol Science, Vol. 18, pp. 511, 1987). From modelingof the evaporation phase of SP, Messing, G., Zhang, S. C. and Jayanthi.G. V., “Ceramic Powder Synthesis by Spray Pyrolysis,” Journal of theAmerican Ceramic Society, Vol. 76, No. 11, 1993, pp. 2707-26, predictsthat hollow shells of different thickness can be obtained depending onthe concentration gradient at the onset of precipitation. If theprecipitate shell is sufficiently permeable, the remaining solvent canbe removed and the hollow shell structure can be preserved.

[0094] The properties of the precursor solution, including thermalcharacteristics, must be known because they can profoundly effect theparticle morphology during the various SP stages. In general, SP studieshave been confined to aqueous precursor solutions of highly solublemetal chlorides and oxychlorides as well as other water-soluble metalsalts such as nitrates, acetates and sulfates (Messing 1993). Very fewstudies have dealt with colloidal precursors, as would be required forhollow clay shells. An understanding of the colloid chemistry of thedispersion is required for shell processing, especially concerning thestability of the dispersion at the high operating temperatures duringSP.

[0095] Hollow shells with porous walls can be produced economically byprocesses such as spray drying and SP; moreover, they can be preparedwith mean diameters on the order of 50-150 microns. Therefore, suchshells could be used to provide air entrainment in mortar or concretewithout the use of chemical air-entrainment agents. The shells could beintroduced as an ingredient in the concrete mix, so that the quantityand size of the air voids would be guaranteed. The shells could bepretreated to impregnate them with a nucleating agent, such asmetaldehyde, to enhance their effectiveness for frost protection.

EXAMPLES

[0096] Metaldehyde possess one of the highest nucleation temperaturesfor ice nuclei in the vapor phase (˜0.4° C., Fukata 1963); however,little is known about the freezing capabilities (ice nucleated from theliquid phase) of metaldehyde. Furthermore, from the cited literature,the mode of preparation and molecular structure of an ice nucleant isknown to have a drastic effect on the nucleating properties. Thus, aneffort to isolate the critical parameters affecting the freezingnucleation capabilities of metaldehyde was conducted. Freezingexperiments were done on metaldehyde-impregnated Vycor glass sampleshaving uniform 100 Å pores.

[0097] Procedures

[0098] (a) Materials

[0099] The metaldehyde (C₂H₄O)_(n) used in this investigation wasmanufactured by Fluka Chemika. The chemical formula of metaldehyde isoften denoted as (C₂H₄O)_(n) since there is a strong tendency for thetetramer units (CH₃CHO)₄ to form long fibers (as discussed earlier).Metaldehyde should not be confused with polyacetaldehyde (Natta, 1961)which has the same unit but an entirely different head-to-tailarrangement (C-O-C connectivity). Metaldehyde was prepared by severaldifferent methods (crushing in mortar and pestle, washing on a Buchnerfunnel with water, dissolving the THF and then precipitating indifferent solvents) to determine optimal nucleating properties. Coumarin(C₉H₆O₂), or 2H-1-Benxopyran-2-one, was also tested for its nucleationproperties since it was cited as a potential freezing nuclei (Fukata1966).

[0100] (b) Vycor Glass Experiments

[0101] Commercial brand Vycor glass from Corning Glass Works was usedduring the experiment. Vycor glass is prepared by melting a homogeneousmixture of sodium borosilicate liquid and then quenching the mixture toa temperature below the coexistence and spinodal curve where it phaseseparates into two interpenetrating phases. The boron rich phases isleached out leaving behind a silica skeleton with a known distributionof pores sizes. The average pore size is ˜100 Å.

[0102] The Vycor glass samples were first crushed into pieces smallenough to fit into the 50 μl DSC pans. The samples were immersed in a30% hydrogen peroxide solution and heated up to ˜70° C. for severalhours to remove any organic impurities absorbed by the glass. A soak indistilled water was then conducted followed by drying in a 50° C. ovenuntil the samples were clear.

[0103] Prior to conducting freezing experiments, dry Vycor samples weresubmerged in distilled water for ˜1 hour to ensure complete saturation.Water absorption capacities of Vycor samples were estimated by heatingsaturated samples in a Perkin Elmer Thermal Gravametric Analyzer (TGA 7)and noting the stabilized weight loss. Vycor samples were impregnatedwith metaldehyde by soaking in a ˜6.5 mg metaldehyde/mL THF solution ata temperature of ˜60° C. for ˜1 hour and then precipitating in a 0° C.water bath (while stirring). When testing the saturation capacities ofthe impregnated samples in the TGA, maximum heating temperatures were˜40° C. Metaldehyde was visualized in the Vycor samples by viewing in aNikon SMZ-U Zoom 1:10 optical microscope with polarizing filters.

[0104] (c) Differential Scanning Calorimetry (DSC)

[0105] DSC scans were performed on a Perkin Elmer Pyris 1 DifferentialScanning Calorimeter with a cooling and heating rate of 1° C./min.Calibration was provided by melting pure water and n-decane samples.

[0106] For nucleation experiments with metaldehyde, approximately 1 mgof sample was placed on the bottom of the pan. A single drop of water(diameter ˜1 mm to 2 mm) was then pipetted over the metaldehyde andgentle rearrangement of the metaldehyde was done to ensure intimatecontact between the water droplet and the nuclei particles. The weightof the sample pan was recorded before and after the DSC run; if anyweight loss occurred, the test was discarded.

[0107] (d) Surface Morphology

[0108] The surface morphology of the metaldehyde samples wasinvestigated by scanning electron microscopy (SEM) on a Philips XL 30FEG-SEM. Samples were carbon coated with a thickness of ˜20 nm prior toviewing.

[0109] Results/Discussion

[0110] (a) Ice Nucleation by Metaldehyde

[0111] As developed in the theoretical section of the investigation, theproperties of ice nuclei will depend on several factors including size,surface contamination, exposure of active sites and age. To isolate someof these factors and their effects on the nucleation capacity ofmetaldehyde, several different preparation methods were devised (seetable). Description of the different preparation routes for metaldehyde.Sample name Description As-received Metaldehyde taken from the bottle.Care was taken not to damage the needles while placing the samples inthe DSC pans. Crushed Metaldehyde crushed in a mortar and pestle.Testing of the sample in the DSC directly followed the crushing. To testthe effects of age on the crushed samples, the crushed samples wasstored in a sealed glass scintillator bottles for ˜1 month. Water washedMetaldehyde washed on a Buchner funnel with a vacuum. Approximately 1 Lof distilled water was used per gram of metaldehyde. Washed/crushedMetaldehyde washed as above then crushed in a mortar and pestle. THFppt/0° C. Crushed metaldehyde dissolved in a ˜6.5 mg metaldehyde/ml THFsolution at ˜60° C., then precipitated in a 0° C. distilled water (whilestirring). Sample was collected on a Buchner funnel. THF ppt/25° C. Sameas above but precipitated in 25° C. distilled water.

[0112] During cooling, in the DSC, the sample will crystallize at atemperature, T_(C), corresponding to the onset of the exothermicfreezing peak. The temperature was determined by the Pyris 1 DSCsoftware, which takes the intersection of the tangent at the inflectionpoint of the freezing exotherm with the baseline of the curve. It isimportant to remember that the nucleation process is a statistical event(based on the fortuitous groupings of water molecules) so multiple runsof a specific sample will give a range of onset temperatures scatteringaround a mean onset crystallization temperature, T_(C,avg). The tablebelow lists the DSC results for the metaldehyde samples, the as-receivedand crushed coumarin samples and distilled water. Onset temperaturesobtained from DSC scans for metaldehyde (MA), coumarin (CO) anddistilled water. Number of Range of Crystallization Sample samplesactivity [° C.] temperature, T_(C,avg) As-received MA  5 −11.5 to −7.6  −9.1 Crushed MA  8 −3.8 to −2.6  −3.3 Aged crushed MA  3 −4.2 to −3.4 −3.8 Water washed MA 10 −9.8 to −7.0  −8.3 Washed/crushed MA  6 −3.7 to−3.2  −3.4 MA THF ppt/0° C.  6 −6.5 to −1.8  −5.0 MA THF ppt/25° C.  5−8.1 to −4.3  −6.1 As-received CO  3 −15.4 to −7.4  −11.1 Crushed CO  5−7.8 to −4.0  −6.1 Distilled water  3 −15.5 to −15.3 −15.4

[0113] It seems that the largest effect in improving nucleationtemperatures is to crush the sample. Crushing will increase the surfacearea, and more importantly, will increase the density of sites(presumably the basal planes in metaldehyde) in the sample. Both freshlycrushed metaldehyde (crushed and crush/wash) samples possessed thehighest onset temperature at ˜3.3° C.

[0114] Aging showed little, if any, change in the nucleating propertiesof the crushed sample. A discrepancy of only ˜0.5° C. separated the agedcrushed and crushed samples.

[0115] Washing the impurities away with water does not seem to have alarge effect on metaldehyde as evidenced by similar onset temperaturesbetween the unwashed and washed samples. The slight increase in onsettemperature of the water washed compared to the as-received sample couldvery well be due to inadvertent crushing of the metaldehyde whencollecting the sample off the Buchner funnel.

[0116] Allowing the crushed metaldehyde to recrystallize in a warm THFsolution will decrease the onset temperature. Quickly precipitating in a0° C. water bath seems to possess better nucleating potential than theslower 25° C. precipitate. Even though the T_(C,avg) for the twodifferent precipitates only differ by ˜1° C., it is noteworthy that the0° C. precipitate was the only sample to possess an onset temperaturegreater than −2° C. Furthermore, the amount of precipitate collectedfrom the 25° C. water bath was very small, as most of the metaldehydetended to stay in solution. The 0° C. precipitate, however, dropped outof solution almost instantaneously in much larger quantities.

[0117] Coumarin followed the same trends as metaldehyde in the sensethat the crushed samples yielded significantly higher onset temperaturesthan the as-received. Solubility in ethanol and hot water (notattempted) certainly makes coumarin attractive in terms of ease ofimpregnating porous shells; on the other hand, coumarin has suspectedtoxic and carcinogenic properties.

[0118] (b) Morphology of Metaldehyde

[0119] Depending on the mode of preparation, metaldehyde will havevarying surface morphologies. Estimated form optical microscopy and SEMimages, the approximate characteristic dimensions of the various formsof metaldehyde can be seen in the table below. Dimensions of variousmetaldehyde forms estimated from optical microscopy and SEM. SampleLength Thickness As-received 500 μm to 1 mm    50 μm to 100 μm THFppt/0° C. 100 μm to 200 μm Less than 10 μm Crushed* Less than 50 μm Lessthan 5 μm  THF ppt/25° C. Less than 10 μm Less than 5 μm 

[0120] (c) Porous Glass Experiments

[0121] At full saturation, the cleaned and dried Vycor glass wasmeasured to absorb water up to ˜49.5% of its weight (0.495 g of water/gof dry Vycor). This measurement was made from heating samples in a TGA(for greater precision) on several Vycor glass samples submerged indistilled water for several days (ensuring full hydration). Furthermore,for the small DSC Vycor samples (˜3 mg), it was found that only 20minutes was required to fully saturate the sample. Thus, since thedegree of saturation will effect the locations of the freezing andmelting peaks in scanning calorimetry experiments (Takamuru, T.,Yamagami, M., Wakita, H., Masuda, Y. and Yamaguci, T., “ThermalProperty, Structure, and Dynamics of Supercooled Water in Porous Silicaby Calorimetry, Neutron Scattering and NMR Relaxation,” Journal ofPhysical Chemistry B., Vol. 101, 1997, pp. 5730-5739), submersion timeswere at least 30 minutes to ensure consistent water contents.

[0122] It was found that metaldehyde could be precipitated in the 100 ÅVycor pores by precipitating a saturated Vycor sample (containing a warmmetaldehyde/THF solution) in a 0° C. water bath. The metaldehyde canclearly be visualized in an optical microscope by a brownish, grain-likeVycor interior. Taking advantage of metaldehyde's crystallinity,polarizing filters can induce a dramatic scattering effect in theimpregnated sample.

[0123] Since metaldehyde is a hydrophobic material, the absorptioncapacities of the impregnated samples were analyzed. Results showed noevidence of a repulsion effect, and in fact, the samples showed anincrease in absorption capacities to ˜65.8% (g water/g Vycor). Theincrease in absorption for the impregnated samples is thought to be aresult of the damage of the porous network due to the extra dying cyclesrequired for the impregnation procedure. This idea is supported by thefinding that repeated submersion and drying of the impregnated samplesshowed further increases in absorption to ˜94.2%.

[0124] Cooling a fully hydrated Vycor sample from 3° C. to −30° C. andheating from −30° C. to 5° C. shows the presence of one sharp freezingpeak and a double melting peak (or sometimes a single uneven broad peak.The difference between the onset freezing peak and the melting peak isthe characteristic nucleation undercooling, ΔT_(c). Freezing ametaldehyde impregnated Vycor sample (by precipitating the warm THFsolution in 0° C. water) also reveals one characteristic freezing and adouble or broad melting peak. The location of the melting peak wasfairly consistent (indicating fairly uniform pore sizes) for allsamples. The freezing peak, however, was shifted to higher temperaturesby ˜10° C. when metaldehyde was present in the pores of the glass. Theaverage undercoooling for the impregnated Vycor was ˜7.1° C. while theunimpregnated samples scattered around a ΔT_(c) of 17.1° C. See FIG. 8.A comparison of the nucleating properties in the pores of unimpregnatedand impregnated Vycor glass samples. Freezing, Melting, Average SampleT_(c) [° C.] T_(m) [C.] ΔT_(c) [° C.] Δ T_(c) [° C.] Unimpregnated −15.0−1.8 13.2 17.1 −21.5 −1.8 19.7 −21.5 −2.4 19.1 −18.8 −2.4 16.4 −18.9−2.2 16.7 Impregnated  −8.5 −1.8  6.7  7.1 −10.3 −2.4  7.9  −9.1 −2.4 6.7

[0125] It is surprising that the metaldehyde in the 100 Å pores of theVycor sample (ΔT_(c)=7.1° C.) behaves relatively similar to themetaldehyde precipitated directly from the warm THF solution in 0° C.water (ΔT=5.0° C.). First of all, the size of the metaldehyde crystalsin the 100 Å pores will certainly be significantly smaller due to thesize constrictions. (Fletcher predicts that the nucleating capacity of aspherical catalyst will decrease precipitously when radii are less than˜100 Å. The surface morphology of the precipitate should also be alteredsince the quenching rate will be slower (controlled by the diffusion ofwater into the pores. Despite these potential hindrances to nucleationin the 100 Å pores, the confined metaldehyde only sees a depression of˜2° C. from the freely precipitated sample.

[0126] (d) Implications of Metaldehyde Nucleation on Frost Action inConcrete

[0127] For impregnated microshells to show any noticeable frost actionimprovements, the shells must initiate ice growth before significantcrystallization and hydraulic pressures can be generated.Crystallization and hydraulic pressures should only induce tensilestresses over 3 MPa when temperatures are ≦−5° C. (corresponding to ar_(BT) of ˜13 nm) and −4° C. (r_(BT) of ˜16.7 nm), respectively. Thefreely precipitated metaldehyde from 0° C. water with an average onsettemperature of about −5° C. and a maximum near −1.8° C. can certainlycompete with these onset temperatures. The freely precipitatedmetaldehyde, rather than the impregnated metaldehyde in the Vycor glass,is used in the comparison since it was argued that the onset temperatureof the 100 Å metaldehyde in the Vycor glass was depressed due to theeffects of the confined geometry in the 100 Å pores. The shells willpresumably be ˜100 μm, thus minimizing any geometric effects innucleation. Furthermore, it is most important for the metaldehyde tonucleate before T_(BT), or the point where crystallization and hydraulicpressures begin to become potentially dangerous. With an onsetnucleation temperature of −5° C., the impregnated shells should be ableto significantly improve the frost action of pastes with a r_(BT)≦13 nmby confining the ice in the voids and removing the percolation event(i.e., the progressive invasion of ice through the pore space)altogether. This implies that the metaldehyde-impregnated shells shouldhave the biggest impact on very fine pastes with low characteristicbreakthrough radii. For pastes with r_(BT)>13 nm, percolation throughthe cement body could occur; however, the crystallization and hydraulicpressures that are generated will probably be below 3 MPa, causinglittle damage to the concrete.

[0128] The impregnated shells would also be integral in the event of adelayed surface nucleation. Since tensile stresses generated fromdelayed surface nucleation can reach as high as ˜8 times that of thestresses generated from spontaneous nucleation (assuming a surfacenucleation temperature at −0° C.), it is important to remove thefreezeable water before the surface nucleation takes place. If theimpregnated shells can consistently nucleate at −5° C., there should bevery little freezeable water by the time the concrete reaches −10° C.Hence, potentially devastating stresses of ˜20 MPa should be avoidedwith use of the shells.

[0129] (e) Ceramic Microshell Design and Function

[0130] Ideally, the ceramic hollow shells will be relatively strong intension (greater than 3 MPa), sufficiently porous (to allow for liquidflows) and able to nucleate ice at maximum temperatures (ideally beforeT_(BT)).

[0131] The strength and porosity of the shell will be stronglyinfluenced by the firing temperature of the ceramic material. Beingrelatively inexpensive, kaolin is an attractive candidate for the shellmaterial. Kaolin clay consists of mainly ordered kaolinite(Al₂SiO₅(OH)₅), with some mica and free quartz. The firing temperaturewill presumably be in the range between ˜700° C. and ˜1000° C., therebyensuring the increased porosity of metakaolin (Al₂O₃.2SiO₂) derived fromthe dehydroxylation of kaolinite at ˜520° C. and the added strength dueto partial densification. At 980° C., metakaolin goes through a seriesof transformations as it rearranges into a spine and then into smallmullite crystals.

[0132] The increased strength of the shells will impart benefits sincecrystals will be able to grow in voids where the surrounding walls cansupport higher tensile stresses before fracture. Thus, with shelltensile strengths potentially greater than 3 MPa, crystallizationinduced failure should be delayed to higher undercoolings. Furthermore,inclusions of the shells could also improve concrete mechanicalproperties such as fracture toughness and impact resistance providedthat the shells have a high aspect ratio (defined as the ratio betweenthe outer radius and the wall thickness) and a larger Young's moduluscompared to the matrix (Liu, J. G. and Wilcox, D. L., “Design Guidelinesand Water Extraction Synthesis Capabilities for Hollow Icrospheres forLow Dielectric Constant Inorganic Substances,” in Hollw and SolidSpheres and Microspheres: Science and Technology Associated with TheirFabrication an Application, Materials Research Society SymposiumProceedings, Vol. 372, Materials Research Society: Pittsburgh, 1995, pp.231-237).

[0133] As mentioned before, increasing the strength of the shellsimplies a decrease in porosity. The shells must be sufficiently porousto allow water to contact the metaldehyde. If water cannot penetrateinto the interior of the voids, the entire purpose of the shells, thatis to remove the freezeable water from the cement pores, will be lost.Ideally, the metaldehyde will line the inner walls of the shell.However, with the intended impregnation by soaking the shells in a warmTHF/metaldehyde solution followed by precipitating in a 0° C. waterbath, there will inevitably be metaldehyde in the pores of the wall. Thehydrophobicity problem, that is the concern of hydrophobing the shell tothe point of total repulsion of water, will probably not be an issue(while not confirmed definitively) as suggested by the impregnated Vycorglass experiments. Even if there is a repulsion effect, there are boundto be pores where metaldehyde is absent, thus allowing water to freelypenetrate into the void.

[0134] The size of the void space in the shells should be large enoughto avoid any undercooling effects (dictated by the Gibbs-Thomsoneffect). This would allow for maximum nucleation temperature by themetaldehyde. The shell size will be on the order of an air void, ˜100 μmin diameter, so size effects will be negligible. With the diameterknown, the concentration of shells can be calculated by requiring atotal void space volume at least equal to the volume of theoretical icein the paste (calculated from the amount of freezeable water in thepaste).

[0135] Once nucleation occurs in the void space, there should be adraining of the water from the paste into the void. Each shell (like anair-void) will have a characteristic sphere of influence. The net volumeof paste intercepted by the individual spheres should cover the entirepaste to ensure total protection. Furthermore, the growing crystalcreates suction in the liquid (which is responsible for migration of thewater to the void), and that reduces the risk of cracking by putting thesurrounding concrete into compression.

[0136] It is understood that the embodiments described herein are merelyexemplary and that a person skilled in the art may make many variationsand modifications without departing from the sprit and scope of theinvention. All such variations and modifications are intended to beincluded within the scope of the invention.

What is claimed is:
 1. A method of protecting a cementitious mixturefrom freeze damage comprising incorporating an amount of air into thecementitious mixture to form air pores in the cementitious mixture, thecementitious mixture including an amount of an air entrainment agent andan effective agent for nucleating ice in the air voids upon the freezingof the cementitious mixture.
 2. The method of claim 1 wherein the waternucleating agent comprises metaldehyde.
 3. The method of claim 2 whereinthe metadehyde consists of tetrameric units (CH₃CHO)₄.
 4. The method ofclaim 2 wherein the air entrainment agent contains a surfactant.
 5. Themethod of claim 2 wherein the metaldehyde is ground to expose thecrystal planes that are most effective for nucleating ice.
 6. The methodof claim 2 wherein the air entrainment composition includes ceramicshells impregnated with metaldehyde.
 7. The method of claim 2 whereinthe amount of air is from about 3% to about 8% by volume.
 8. A method ofprotecting a cementitious mixture from freeze damage comprisingincorporating an amount of air into the cementitious mixture to form airvoids in the cementitious mixture, the cementitious mixture including anamount of an air entrainment composition and an effective amount of anagent for nucleating ice, selected from the group of metaldehyde,acetoacetanilide, p-bromoacetphenone, coumarin, m-nitoaniline, pthalicanhydride, and 2,4,6-trichloraniline for nucleating ice in the airpores.
 9. The method of claim 8 wherein the air entrainment compositioncomprises ceramic shells impregnated with an agent for nucleating ice.10. The method of claim 8 wherein the air entrainment compositioncomprises glass ceramic shells impregnated with an agent for nucleatingice.
 11. The method of claim 8 wherein the air entrainment compositioncomprises shells of kaolin impregnated with an agent for nucleating ice.12. The method of claim 8 wherein the air entrainment compositioncomprises clay shells inpregnated with an agent for nucleating ice. 13.A method for protecting concrete from freeze damage comprising: mixingan air entrainment composition into a cementitious mixture; adding anagent for nucleating ice to the cementitious mixture; and allowing thecemetitious mixture to form concrete.
 14. The method of claim 13 whereinthe agent for nucleating ice is added to the air entrainment compositionbefore mixing the air entrainment composition with the cementitousmixture.
 15. The method of claim 14 wherein the agent for nucleating icecomprises metaldehyde.
 16. A concrete composition including pores andair voids comprising a nucleating agent within the air voids fornucleating ice.
 17. The composition of claim 16 wherein the nucleatingagent comprises metaldehyde.
 18. A method of forming porous shells foruse as an air entrainment composition for concrete comprising:suspending particles in a slurry; atomizing the slurry to form droplets;drying the droplets; and sintering the dried droplets to form shellswith porous outer walls.
 19. The method of claim 18 wherein theparticles comprise ceramic material.
 20. The method of claim 18 whereinthe particles comprise clay.
 21. The method of claim 18 wherein theparticles comprise aluminum oxide.
 22. A method of protecting concretefrom freeze damage by providing an air entrainment agent comprisingmixing an amount of hollow porous shells into a cementitious mixture andallowing the cementitious mixture for form concrete, wherein the hollowporous shells allow water to pass thereinto.
 23. A concrete compositionformed of a cementitious mixture with porous hollow shells mixed thereinfor allowing water to pass thereinto, to prevent freeze damage to theconcrete composition.
 24. The composition of claim 23 wherein the poroushollow shells are formed of a ceramic material.
 25. The composition ofclaim 23 wherein the porous hollow shells are formed of clay.